In many applications, traditional digital modulation schemes provide adequate performance. However, there exists other applications for which modulations having greater flexibility and sophistication are absolutely essential. Digital modulation techniques which yield the needed greater flexibility, require the use of digital-to-analog converters with large bandwidth capability. An example of an application requiring increased flexibility may be a satellite channel over which it is desired to simultaneously send two independent and generically different types of information. In this instance, a modulation scheme capable of handling two asynchronous data streams, with different rates, different powers and different formats, would be required. Another example of an application requiring increased flexibility to accommodate increased sample rates and bandwidths is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95A (TIA/EIA IS-95A) herein after referred to as IS-95A. In accordance with IS-95A, the coded communication signals used in the DS-CDMA system comprise signals that are transmitted in a common 1.25 MHz bandwidth, hence, spread-spectrum, to base sites of the system from communication units, such as mobile or portable radiotelephones, that are communicating in the coverage areas of the base sites. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum is commonly referred to as a carrier frequency, capable of conveying multiple sync, paging and digital voice channels associated with a CDMA communication signal.
Complex digital modulation schemes, such as those required in a DS-CDMA system, require the transmitter to be implemented using quadrature modulation techniques. Quadrature modulation circuits are known in the art. Quadrature modulation circuits utilize two digitally encoded data streams to amplitude modulate independently, a sine and cosine component of a carrier signal. The two digitally encoded data streams are referred to as an in-phase (I) signal and a quadrature (Q) phase signal. The I signal mathematically represents a real component of the baseband version of the final resultant modulated signal while the Q signal mathematically represents an imaginary component of the baseband version of the final resultant modulated signal. The sum of the I and Q signal results in the creation of a unique set of two-dimensional signal vectors or symbols.
In a DS-CDMA transmitter which utilizes the quadrature modulation technique, the in-phase (I) and quadrature (Q) phase component signals are used to amplitude-modulate the sine and cosine components of a carrier signal generated by a local oscillator as follows. First the in-phase (I) and quadrature (Q) phase signals are converted into amplitude samples, N bits wide, via corresponding amplitude converters. Next, two digital-to-analog (D/A) converters are utilized to convert the I and Q digital amplitude samples into corresponding analog signals which are input to corresponding signal conditioning circuits and then used to amplitude modulate the sine and cosine components of the carrier signal. The ultimate goal is to transmit an error-free signal.
Obviously, optimum quadrature modulation performance is obtained via D/A signal conversion across a large bandwidth. The need for a large bandwidth capacity combined with the desire to generate an error free signal requires the use of D/A converters with a high bit resolution capable of accommodating the high sample rates and bandwidths associated with wide-band cellular systems such as DS-CDMA. Unfortunately, the cost of the wide-band D/A converters are exponentially proportional to their required bit resolution. For example, a bit D/A converter has a complexity factor and cost proportional to 2.sup.9 while an 8 bit D/A converter has a complexity factor and cost proportional to 2.sup.8. Thus a reduction of one bit in the bandwidth resolution of a D/A converter reduces it's complexity with respect to the number of gates required, thereby considerably reducing the cost of the device.
Therefore a need exists for a method and apparatus for performing a modified quadrature modulation of a digital signal which reduces the required bit resolution of a D/A converter in a quadrature modulation circuit while still maintaining the bandwidth capacity of the D/A converter.